Products normally used in the production of soilless mixes for greenhouse crop production are often water repellent (hydrophobic). The hydrophobic surface chemistry of these materials, particularly those from organic sources, is somewhat peculiar and does not relate well to other surfaces. For example, sphagnum peat moss, hypnum peat moss, reed-sedge peat, composted bark and rockwool often become very difficult to wet which defeats the purpose for which they were originally intended. The difficulty in wetting increases as the mix dries out, which can occur in storage, after preparation, or when used to pre-fill pots and flats by the grower and stored for some time before use. This situation presents many problems to the grower because getting the mix evenly wetted is of paramount importance to the healthy growth of plants or seeds that are grown in the mix.
A possible answer to this problem is to add an appropriate surfactant to the mix to reduce surface tension between the applied water and the hydrophobic mix components. But, the use of materials currently in commerce is not entirely satisfactory. Relatively high rates of these materials are required to get good wetting properties of the mix and these rates can be injurious to crop growth. Furthermore, the conventional dry formulations of these materials, which are often based on corn cob grits and vermiculite, do not work well because the surfactant must first be dissolved from these carriers and bind to the hydrophobic surfaces of the peat, rockwool or bark before they become activated. Consequently, several waterings are normally required before they become active. Furthermore there is an environmental concern because the described compounds contain nonyl-phenol ethoxylates or other mono or di-alkyl phenol ethoxylates. The conjeners of these compounds are known to bioaccumulate in the biosphere, they have a deleterious affect on fresh water crustaceans, and their use has been banned in many countries.
The majority of the surfactants currently discovered and developed have been designed to give fast wetting because they are used as cleansers, soaps and detergents. Fast wetting, however, is not necessarily desirable because surfactants would have to be applied with each watering. What is more desirable is rewetting of the peat or other substances following treatment with a surfactant giving residual activity.
Unfortunately many such surfactants can also have biological activity. They can, in fact, be detrimental to plant growth and many are actually fatal and can act as herbicides. Others have other biological activity and can cause death to insects. Many of these compounds have been previously tested and were found to interfere with seedling growth. Some surfactants act by disrupting membrane integrity as the hydrophobe bonds to the phospholipid membrane of the cell wall. Others can act as plant growth regulators and, while their use may be beneficial in achieving certain results, they cause undesirable effects in greenhouse crop production.
Furthermore, if the selected surfactant is to remain active in the mix beyond the initial watering, it is desirable that the hydrophobe of the surfactant bond tightly to the hydrophobic mix component and not leach out when plants are repeatedly watered.
In addition, it is highly desirable that surfactants be compatible with fertilizers and other water soluble chemical charge stocks used in preparation of the final horticultural mix.
Although surfactants may be applied as a liquid, it is also desirable that they may be presented in a dry formulation for ease of application and use by the grower. This creates an additional problem in that most common carriers, such as vermiculite, corn cob grits, clay and so forth, also absorb the surfactant or tightly bind them to their surfaces which makes it necessary for the surfactant to first be dissolved from the carrier before it can become active.
Although there are over 4000 surfactants currently in commercial use, the discovery of a surfactant for use in soilless mixes is not a trivial problem because it must meet certain specific and difficult criteria, namely:
(1) Be compatible with the surface chemistry of peat, rockwool and bark, PA1 (2) Safe for plant growth, particularly seed germination and seedling growth, PA1 (3) Bind tightly to mix components so that it is not leached from the matrix following watering and have residual activity for a full normal cropping season, PA1 (4) Give not only fast initial wetting but also and more importantly rewetting after application to the mix components, PA1 (5) Be compatible with fertilizers and other chemical components and PA1 (6) Have ease of application and be effective in both a liquid and solid formulation.